Tuesday, September 20, 2011

The Siberian (or Amur) tiger is the largest of all the cats. Once numbering in the thousands, their population has dwindled to less than 400, mostly along a narrow range of mountains in southeastern Siberia, along the Sea of Japan.
With a massive build, powerful limbs, and large canine teeth, these apex predators are built to hunt. Siberian tigers have long retractable claws for catching their prey which are pulled back into a protective sheath when walking to keep them razor sharp. They prey on the deer and wild pigs that live in their territory, which ranges from 250 to 450 square kilometers for an adult female.

A large male Siberian tiger can reach lengths of

up to 3.3 m long and weigh as much as 300 kg,

almost twice as large as an average adult lion.

Tigers are nocturnal creatures for the most part, but the Siberian tiger is also active during the day. They have keen eyesight which can pick up even the slightest of movements and their hearing and sense of smell is also quite sensitive. Female tigers have litters of two to four cubs every few years. The cubs are nursed for five or six months before they are old enough to leave their den and join mom on hunting trips. After a year they can hunt for themselves and by the time they reach three to five years old they will strike out on their own. The Siberian tiger’s future is uncertain. In addition to the pressures from poaching and loss of habitat, researchers have concluded that the sub-species has fallen below the critical threshold where its genetic diversity can sustain a healthy population. The genetic base of the Siberian tiger is less than previously thought, with an effective population of only 14 individuals. Effective population is a measure of the genetic diversity of a species. So even though the actual population is stable or climbing, genetically-speaking this sub-species is nearly extinct. Is it just a matter of time before the Siberian tiger joins the Javan, Caspian and Bali tiger sub-species in extinction? Not if members of the Siberian Tiger Project have a say. Since 1992 the Wildlife Conservation Society has supported the Siberian Tiger Project, a dedicated group of scientists that use radio-telemetry to monitor more than 60 tigers to help with research and conservation efforts. Wildlife biologists have studied their habits, reproduction, and how they interact with other species, especially humans. Current research is focused on cub mortality and their struggle to survive to adulthood. They have learned that young tigers may roam over 200 kilometers to find new territory when they leave their mother’s care. Researchers are also trying to combat poaching and understand its effect on tiger population. They have estimated that four out of five tiger deaths are caused by humans. They realize it is vitally important to protect female tigers so that they can live long enough to ensure the survival of their cubs to the next generation. If you would like to learn more about the Siberian Tiger Project and their efforts, watch this documentary.

Saturday, September 17, 2011

Light refracted through a prism. The shorter the wavelength, the more the light refracts, which is what causes it to separate into its spectral colors.

Last week we learned that the electromagnetic spectrum encompasses much more than the visible light represented by the colors of the rainbow. In fact, visible light is less than one percent of the entire spectrum which includes radio wave, microwaves, infrared, ultraviolet, X-rays and gamma rays. As the name indicates, electromagnetic waves are composed of both electric and magnetic fields which oscillate perpendicular to one another and to the direction of travel. In the vacuum of space, these waves travel at the speed of light until they interact with matter. When light interacts with matter it can do one of several things, depending on its wavelength and what kind of matter it encounters: it can be transmitted, reflected, refracted, diffracted, adsorbed or scattered.

The simplest interaction with light is transmission, which occurs when light passes through the object without interacting. Light coming through window is a simple example of transmission.Reflection occurs when the incoming light hits a very smooth surface like a mirror and bounces off, like a mirror. Refraction occurs when the incoming light travels through another medium, from air to glass for example. When this happens the light slows down and changes direction. This change in direction is dependent on the light’s wavelength so its spectrum of wavelengths are separated and spread out into a rainbow.Diffraction occurs when light hits an object that is similar in size to its wavelength. When light passes through a sufficiently-thin slit it will diffract and spread. If it’s visible light, this will also create a rainbow. Absorption occurs when the incoming light hits an object and causes its atoms to vibrate, converting the energy into heat which is radiated. Anyone with a dark-colored car on a hot day will experience the effects of adsorption. Scattering occurs when the incoming light bounces off an object in many different directions. A good example of this is known as Rayleigh scattering, where sunlight is scattered by the gasses in our atmosphere. This is what gives the sky its blue color.

Saturday, September 10, 2011

The relationship between frequency and wavelength in EM waves (c is the speed of light).

Electromagnetic energy travels in waves that span a broad spectrum. These electromagnetic (EM) waves are ubiquitous, and without them life as we know it would not exits. EM waves form the foundation of the information age and the technologies we use every day: radio, television, consumer electronics, remote controls, cell phones, microwave ovens, X-rays and other medical technologies all use EM waves. EM waves transmit energy, but unlike water or sound waves, they do not need a medium to travel through. They move through the vacuum of space at the speed of light (about 300,000 km/s). EM waves have crests and troughs like all waves. The distance between consecutive crests is the wavelength. The number of crests that pass though a given point in one second is the wave's frequency which has a unit of measurement called the hertz (Hz), named after the great German physicist Heinrich Hertz. Hertz was the first to demonstrate the existence of EM waves and the first to send and receive radio waves. Radio waves, which have the longest wavelengths, are at the low-energy end of the spectrum and at the high-energy end of the spectrum, with the shortest wavelengths, are gamma rays. The other bands in between, ordered by decreasing wavelength, include microwaves, infrared, visible light, ultraviolet and X-rays. Its ironic that the band of radiation with the greatest significance to us—visible light—represents only a tiny fraction of the spectrum, from about 390 to 750 nanometers. Compare that to radio waves which range from a few millimeters to a kilometer in length or more. Our atmosphere filters out a large portions of the electromagnetic spectrum, but thankfully visible light passes through mostly unaffected.

The range of wavelengths in the electromagnetic spectrum compared to sizes found in everyday life.

The frequency of any given EM wave is inversely proportionate to its wavelength: EM waves with long wavelengths have lower frequencies and EM waves with short wavelengths have higher frequencies. The equation that governs this relationship is ƒ = c / λ, where ƒ is the frequency, c is the speed of light, and λ (lambda) is the wavelength. So for a radio wave with a wavelength of one kilometer, its frequency would be 300,000 Hz or 300 kHz. In the United States, AM radio is broadcast at frequencies between 520 kHz and 1610 kHz. Using our formula, we can find that they correspond to wavelengths ranging from 186 to 577 meters. FM radio broadcasts at much higher frequencies: 87.8–108 megahertz (MHz). One megahertz is equal to 1,000 kilohertz, so that would correspond to wavelengths in the range of 2.7–3.4 meters—quite a difference!

Over the next few weeks we will talk more about the various aspects of the electromagnetic spectrum, so if you have any questions related to this topic, let me know by posting them on my Facebook page.

Monday, September 5, 2011

A graph of freezing rates for two water samples demonstrating
the Mpemba effect. The gray line is water at 42.9°C and the black
line is water at 18.6°C. Note the hotter water spends considerably
less time in the 0–4°C range, likely due to convection.
Source: picotech.com.

As odd as it seems, sometimes warmer water freezes faster than cooler water. This observation is known as the Mpemba effect, names after Erasto Mpemba from Tanzania. Even though the effect had been known for years to layman such as plumbers and ice rink workers, the scientific community had not taken the claim seriously. But Mpemba, a high school student at the time, provoked a visiting physics professor to test his claim and to his surprise it held true. They published the results jointly in 1969. The Mpemba effect is only observed under certain conditions and there are many factors which can effect how quickly water cools. Conduction is often the biggest factor. Take two similar containers, one containing cold water and the other containing an equal amount of hot water and place them both in the freezer. The hot container will melt any ice on the freezer shelf that it comes in contact with and refreeze as the container cools, creating a good thermal contact between the freezer shelf and the hot water container which will remove heat from the warmer water more quickly. On the other hand, the cooler water container will just sit on the layer of poorly-conducting freezer frost, so it will take longer to cool down. This is a pretty good explanation but even if you control for it by insulating the contact points to the freezer you can still observe the effect. The explanation most-often cited is convection. As the warmer water rapidly cools at the surface, convection currents will develop and create an uneven distribution of temperature with hot water nearer the surface and therefore more evaporation from hot water than from cold. More evaporation means less mass to freeze so it would cool faster. And while this is probably the biggest contributing factor it’s not the only mechanism that drives the Mpemba effect, as not enough water would be lost to evaporation to account for the difference.Supercooling is another possible factor that contributes to the phenomenon. Initially cooler water is thought to dip farther below 0°C before freezing, but the mechanism for this is not well understood. Another factor could be the amount of dissolved gases which would be less in water that has been heated. But the most probable conclusion is that there just isn’t any single unique explanation as to why hot water sometimes cools more quickly than cold.